Parse [ after nullable type as part of the type

[cston]

Parse ? [ as a nullable array type always in switch and is expression pattern types: x switch { string ? [] y => y }

As a result, parentheses are required when the [ starts a lambda expression with attributes: x is string ? ([A] y => y) : z

Fixes #55013.

[cston]

                        return !CanStartExpression();

Before this change, CanStartExpression() returned true for [ in x switch { string?[] y => y } so [] y => y was treated as a lambda expression with attributes rather than a well-formed switch expression arm.

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In reply to: 885021921

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Refers to: src/Compilers/CSharp/Portable/Parser/LanguageParser.cs:6861 in 4b559f9. [](commit_id = 4b559f9, deletion_comment = False)

[cston]

@dotnet/roslyn-compiler, please review this fix for parsing nullable array types in switch and is patterns. The actual parsing change is small, but there are a number of parsing tests included. Thanks.

[jcouv]

Makes me glad that we decided to separate list-pattern from the more general recursive pattern ;-)

[jcouv]

LGTM Thanks (iteration 2)

[jcouv]

Approved by BU and Division triage in email. Can be merged to 16.11
FYI @allisonchou